Preparation of unsaturated aldehydes from propargyl alcohol and conjugated diolefins

ABSTRACT

Unsaturated aldehydes are prepared by mixing propargyl alcohol, a catalyst and a conjugated diolefin and heating the mixture for a time and at a temperature sufficient to form an unsaturated aldehyde. The diolefin may be 2-methylpentadiene, 2(5,5 dimethyl 5 hydroxy pentyl)1,3 butadiene or myrcene. The catalyst may be inorganic, organic, organometallic or mixtures thereof.

This application claims the benefit of U.S. Provisional Application No.60/082,324 filed Apr. 20, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to the synthesis of unsaturated aldehydesfrom acetylenic alcohols and conjugated diolefins. More particularly, itrelates to the synthesis of unsaturated aldehydes from the reaction ofpropargyl alcohol with conjugated diolefins.

Numerous 2+4 cycloadditions have been known since long before the adventof the pericyclic theory; they are among the most powerful of syntheticreactions. The most important of these is the Diels-Alder reaction.Although the Diels-Alder reaction occurs in the unsubstituted case, itis most successful when the diene and the alkene (referred to in thiscontext as the dienophile) bear substituents of complementary electronicinfluence. Although these are most commonly an electron-donating groupon the diene and an electron-withdrawing group on the dienophile, thereare also a number of instances that illustrate inverse electron demand,that is electron-withdrawing groups on the diene and donating groups onthe dienophile.

Unsaturated aldehydes can be prepared by the Diels-Alder reaction. Anexample of such a preparation is the reaction of acrolein withconjugated dienes. Acrolein is an α, β-unsaturated carbonyl compoundwhich is a highly toxic lachrymatory liquid and which has a low boilingpoint (53° C.). Examples of the preparation of unsaturated aldehydes byreaction of acrolein with conjugated dienes are the preparation ofCyclal C (reaction 1 below); the preparation of myraldene (reaction 2below); and the preparation of cyclohexal (reaction 3 below). ##STR1##

Unfortunately, the highly toxic nature and low boiling point of acroleinmake it a very dangerous compound. The fumes of acrolein are copiouslyproduced at normal temperatures, i.e., above 0° C. For this reason,restrictions have been placed on transporting acrolein, and acrolein isno longer generally available. However, the need to prepare unsaturatedaldehydes still remains and is very important in many industries, suchas in the fragrance industry.

Propargyl alcohol is a compound which is less toxic and has a higherboiling point (114-115° C.) than acrolein. Propargyl alcohol can beconverted to acrolein under particular reaction parameters. Catalystsused for such transformations include vanadium compounds, such as thosewhich are discussed in Chabardes, et al., British Patent Specification1,204,754, and a combination of titanium and copper catalysts, such asthose described in Chabardes, et al., U.S. Pat. No. 4,749,814, thedisclosures of each of which are incorporated herein by reference. Thereare, however, disadvantages to the reactions disclosed in thesedocuments, which include, for example, that under the disclosed reactionparameters, acrolein polymerizes readily even in the presence of aninhibitor such as hydroquinone thereby limiting the useful yield ofacrolein.

SUMMARY OF THE INVENTION

We have now found that unsaturated aldehydes can be synthesized frompropargyl alcohol in the presence of a conjugated diolefin. The reactionprocess of the present invention employs a reactant which is readilyavailable, less toxic and which has a higher boiling point than acroleinto allow for safe transportation. In addition the synthetic method ofthe present invention avoids the direct use of acrolein in preparingunsaturated aldehydes, while maintaining the ability to prepareunsaturated aldehydes in significant yields.

In this reaction which occurs in a one-pot system, propargyl alcohol isconverted to acrolein in the presence of a conjugated diene, and uponconversion to acrolein, the acrolein immediately reacts, as soon as itis formed, with the diene to produce an unsaturated aldehyde. Proceedingin accordance with the present invention, allows one to avoid low yieldswhich occur in the reactions disclosed in the documents noted above, byavoiding polymerization of acrolein. It is also possible to obtainhigher yields of the desired unsaturated aldehydes than would beotherwise obtainable by a normal two step synthesis. A further advantageof this process, is that it avoids the potential toxic dangers ofacrolein because the acrolein formed is an intermediate which only has atransient existence within the vessel in which the reaction takes place,and is present at a very low concentration before it further reacts toform the unsaturated aldehydes.

DETAILED DESCRIPTION OF THE INVENTION

The unsaturated aldehydes of the structures (I) and (II) which can beformed by the present synthetic method as illustrated in equations 4 and5 below, are perfumery ingredients or intermediates for perfumeryingredients. In accordance with the present invention, propargyl alcoholis combined with a conjugated diolefin and the components are mixed andheated in the presence of a catalyst which isomerizes the propargylalcohol to acrolein (as illustrated in reaction 4) which reacts with theconjugated diolefin in a Diels-Alder reaction to give the desiredunsaturated aldehyde products (as illustrated in reaction 5). ##STR2##

In the unsaturated aldehyde products of reaction (5), R¹, R², R³, R⁴,R⁵, and R⁶ may be the same or different, and may be hydrogen, an alkylradical (CnH₂ n+1), a substituted alkyl radical (CnH₂ n X where X canbe, for example, OH or OR), an aryl radical (C₆ H₅) or a substitutedaryl radical (such as C₆ H₄ X or C₆ H₃ XY where X and Y are substituentssuch as OH and OR) which, in turn, depends on the diolefin employed.

Where R¹ -R⁶ are alkyl radicals, R¹ -R⁵ preferably are methyl, ethyl orpropyl, and R⁶ preferably is methyl, ethyl or 4-methyl-3-pentyl. WhereR⁶ is a substituted alkyl radical, it preferably is4-methyl-4-hydroxypentyl or 4-methyl-4-methoxypentyl. Where R⁶ is asubstituted aryl radical, it preferably is phenyl, 2-hydroxyphenyl,2-methoxyphenyl, 4-hydroxyphenyl or 4-methoxyphenyl.

Other acetylenic alcohols which may be employed are indicated by thegeneral formula shown below wherein when R¹ and R² are hydrogen, R³ ispreferably methyl. When R² and R³ are hydrogen, R¹ is preferably methyl.##STR3##

Diolefins which may be employed in the process of the present inventioninclude 2-methylpentadiene, myrcene and 2(5,5 dimethyl 5 hydroxypentyl)1,3 butadiene.

Solvents which may be used in the process include hydrocarbon solvents,preferably having boiling points of above about 200° C. A solventpreferably employed is dixylylethane.

Catalysts which may be employed in carrying out these processes includeinorganic and/or derivatives thereof; organic and/or derivativesthereof; and organometallic catalysts. The catalysts may be employedalone or in combinations.

The process may be carried out at a temperature sufficient to initiatethe reaction. The temperature may be of from about 100° C. to about 150°C. More preferably, the temperature of the reaction is from about 120°C. to about 140° C., and most preferably it is from about 125° C. toabout 135° C.

The time required for the reaction to take place is not critical and asufficient time may be from a few seconds (that is, the reaction occursimmediately), to as long as a day. Generally, the reaction occurs over aperiod of from 30 minutes to 4 hours, and more particularly from 2 hoursto 4 hours. The time for the reaction, of course, depends on thetemperature at which the process is carried out and on the particularcatalyst employed in the process.

The amount of propargyl alcohol and of conjugated diolefin which may beemployed, will depend on the amount of unsaturated aldehyde desired tobe produced, based on the yield of unsaturated aldehyde that thereaction provides.

EXAMPLES

The present invention is described further in the following exampleswhich are presented solely for the purpose of providing a furthernon-limiting illustration of the invention. In the following examples,yields of the products are based on the amount of propargyl alcoholemployed in the processes.

Example 1

This example illustrates the formation of an unsaturated aldehyde underconditions of autogeneous pressure.

3.1 g of titanium isopropoxide, 1.5 g of cuprous chloride, 18.0 g ofp-toluic acid, 19.4 g of dixylylethane, 56.0 g of propargyl alcohol, and82.0 g of 2-methylpentadiene were added to a 500 ml autoclave undernitrogen atmosphere and heated at autogenous pressure to a temperatureof 130° C. for 20 hours. The resulting product mixture was then analyzedby gas chromatography using calculations based on dixylylethane as theinternal standard. The analysis revealed a 92% conversion for thepropargyl alcohol, a 70% conversion for the methylpentadiene, and ayield of Cyclal C of 45.4% which was acceptable.

Example 2

This example illustrates the formation of an unsaturated aldehyde underatmospheric pressure.

0.80 g of titanium isopropoxide, 0.40 g of cuprous chloride, 3.5 g ofp-toluic acid, and 10.1 g of dixylylethane were added to a 100 ml 3-neckround bottom flask arranged and fitted with a delivery funnel, athermometer, and a reflux condenser and having a magnetic stirrercontained within the flask. Propargyl alcohol (11.1 g), myrcene (39.1 g)and dixylylethane (10.0 g) were added to a separatory funnel and thefunnel was then shaken to form a mixture. Pressure was intermittentlyreleased from the funnel until most or all of the pressure from thereaction in the funnel had been released. The funnel was then left tostand until an upper layer phase and a lower layer phase were formed andseparated out from the mixture. The lower layer was then released fromthe funnel and transferred to the delivery funnel fitted to the flask.The upper layer was transferred directly into the flask.

The reaction components in the flask were then heated and mixed undernitrogen atmosphere to a temperature of 130° C. and the lower layer wasthen added dropwise from the delivery funnel into the mixture untilreflux was noted at the condenser. Further addition was made graduallyso that a constant temperature of 130° C. was maintained. The total timefor addition of the lower layer from the delivery funnel was about threehours. After a further twenty minutes, a sample of the reaction productwas taken from the flask, showing an acceptable yield of 45.5% myraldeneby internal standard. The conversion of propargyl alcohol and myrcenewere 93% and 71%, respectively.

Comparative Example

This comparative example illustrates a reaction which is carried out intwo steps. In this example, no reaction occurs at the reflux temperatureof the propargyl alcohol (114-115° C.) but the reaction may be effectedby adding the propargyl alcohol gradually to the catalyst in a highboiling solvent at 130° C. To optimize the yield of acrolein, it wasdistilled off as formed. In spite of this the yield of acrolein was only22%.

2.5 g of titanium isopropoxide, 1.2 g of cuprous chloride, 80.0 ml ofdixylylethane, and 18.8 g of 4-methoxycinnamic acid were combined andmixed with a magnetic stirrer while heating the mixture to 130 E in 3neck round bottomed flask. The flask was also fitted with a distillationcolumn and a receiver, thermometers and a nitrogen bypass. Propargylalcohol was added slowly to the mixture in the flask at a rate such thatthe distillate showed an acrolein content of at least 80-90%. After theaddition of approximately 40 ml propargyl alcohol, there was no furtherconversion indicating that the catalyst had been deactivated. Thereaction resulted in an overall conversion of propargyl of 74% and ayield of 22% acrolein based on conversion. In the second step, thereaction of acrolein with the conjugated diolefin normally occurs in ayield of around 50%.

As can be seen, the single step reaction of the present invention avoidsthe problems attendant the two step reaction, but yet achievescomparable yields of products.

While the invention has been illustrated and described with respect toillustrative embodiments and modes of practice, it will be apparent tothose skilled in the art that various modifications and improvements maybe made without departing from the scope and spirit of the invention.Accordingly, the invention is not to be limited by the illustrativeembodiments and modes of practice.

We claim:
 1. A process for preparing an unsaturated aldehyde comprisingmixing propargyl alcohol, a catalyst system comprising (1) a titaniumderivative, (2) a copper derivative and optionally (3) an inorganic ororganic acid and a conjugated diolefin and heating the mixture to forman unsaturated aldehyde.
 2. A process according to claim 1 wherein thediolefin is selected from the group consisting of 2-methylpentadiene,myrcene and 2-methyl-6-methylene-7-octen-2-ol.
 3. A process according toclaim 1 where in the catalyst is selected from the group consisting ofinorganic, organic, organometallic and mixtures thereof.
 4. A processaccording to claim 2 where in the catalyst is selected from the groupconsisting of inorganic, organic, organometallic and mixtures thereof.5. A process according to claim 4 wherein the catalyst is an inorganiccatalyst.
 6. A process according to claim 3 wherein the process iscarried out at a temperature of from about 100° C. to about 150° C.
 7. Aprocess for preparing an unsaturated aldehyde of the formula: ##STR4##wherein R¹, R², R³, R⁴, R⁵, and R⁶ are selected from the groupconsisting of hydrogen, alkyl radicals, substituted alkyl radicals, arylradicals and substituted aryl radicals, comprising mixing propargylalcohol, a catalyst system comprising (1) a titanium derivative, (2) acopper derivative and optionally (3) an inorganic or organic acid and aconjugated diolefin and heating the mixture to form the unsaturatedaldehyde.
 8. A process according to claim 7 wherein the diolefin isselected from the group consisting of 2-methylpentadiene and myrcene. 9.A process according to claim 7 wherein the catalyst is selected from thegroup consisting of inorganic, organic, organometallic and mixturesthereof.
 10. A process according to claim 8 wherein the catalyst isselected from the group consisting of inorganic, organic, organometallicand mixtures thereof.
 11. A process according to claim 10 wherein thecatalyst is an inorganic catalyst.
 12. A process according to claim 5wherein the process is carried out at a temperature of from about 100°C. to about 150° C.